The polymerization of ethylene is carried out with coordination catalysts of various types, one of the most known being the reaction product of a titanium or vanadium compound with a metallorganic compound of groups I, II and III of the periodic system.
As is known, the use of vanadium catalysts permits to obtain, differently from the Ti catalysts, polyethylene with a rather wide molecular weight distribution (DPM).
From FR Pat. No. 2,378,047 components of catalysts for polymerizing ethylene are known, which are prepared by reacting vanadium halogenated compounds having a valence higher than 3 (for example VCl.sub.4, VOCl.sub.3) with an aluminium alkyl in the presence of an ether.
The reaction product comprises or essentially consists of VCl.sub.3.
The catalysts obtained from said catalyst components by reaction with an aluminium alkyl have a rather high activity, although not so high as to lead to the forming of polymers having a low content of catalyst residues, in particular having residual V contents lower than a few ppm.
In fact it is known that polymers containing V in amounts higher than a few ppm are physiologically prejudicial, wherefore the polymers obtained with said catalyst had to be subjected to long and expensive purification processes.
In the field of elastomeric copolymers the polymerization of ethylene and of alpha-olefins in the presence or in the absence of diolefins has been effected till now with catalysts of different types: one of the most used is composed by the reaction product of a vanadium compound and by a metallorganic compound of the metals of groups I, II and III of the Periodic System.
Said copolymerization is preferably effected by using catalysts which are soluble and highly dispersible in the polymerization medium such as: vanadium halides and oxyhalides, alcoholates and acetylacetonates of vanadium and of vanadyl.
By using these catalysts it is possible to obtain substantially amorphous copolymers, characterized, in the vulcanized state, by precious elastomeric properties. The polymer yields referred to the catalyst employed, however, are not so high as to lead to the forming of polymers having a low content of catalyst residues, in particular having a low content of residual vanadium.
Attempts have been made to obviate this disadvantage by employing activating substances, but also in such case the yields remain always very low and do not permit to avoid purifying the polymer from the catalyst residues.
Therefore the need was felt of having available V catalysts having a high activity and enabling to obtain ethylene polymers containing such percentages of V as to avoid any purification of the polymer, and simultaneously enabling to obtain a polymer with a broad DPM.